Nitrogen-13 is commonly used in scanning operations where an ammonium solution containing this isotope is introduced into the body, and the distribution monitored by state-of-the-art techniques. It is desirable to produce the nitrogen-13 by a relatively simple process so as to limit the cost of this medical technique. One known prior art method teaches the use of natural water in a batch or recirculating mode to produce predominantly nitrogen-13 oxides by proton irradiation. These oxides must be chemically reduced in a basic solution to ammonia which is then distilled and collected. Prior devices and methods employing this approach produce added complexity, chemical losses and processing time with concomitant crucial radioactive decay loss. Using this method in a large cyclotron (16 MeV) at about 20 .mu.A of protons, about 175 mCi of ammonium ion is available in a time period of about twenty-five minutes after the initiation of bombardment.
A second known prior art method, as described in our U.S. Pat. No. 4,752,432 issued Jun. 21, 1988, teaches the use of direct production of nitrogen-13 ammonium ions in an aqueous carbon-13 slurry target. This process requires the use of expensive carbon powder highly enriched in carbon-13. Thus, it is limited in production to the use of low proton beam currents in order to prevent the carbon from stopping the flow through the target by plugging porous metal frits used to contain the powder. Using a cyclotron of about 11 MeV at about 3 .mu.A, about twenty-five mCi of ammonium ion is produced in about 13 minutes.
A third known prior art method teaches direct production of nitrogen-13 ammonium ion in a target filled with natural water and maintained under a reducing overpressure of ten atmospheres of hydrogen gas. Reproducible production using this method requires the use of thoroughly degassed deionized target water and, in addition, the flushing of the target lines and the target with hydrogen gas just before loading and running the system. If these procedures are not carefully followed, experience has shown that most of the nitrogen-13 activity produced will be in the form of nitrogen gas and not ammonium ion. This method is also limited to 20 .mu.A beam currents of 18 MeV protons. Experience has shown that if the proton current is above 20 .mu.A, most of the nitrogen-13 produced will be in the form of nitrogen oxides and not ammonium ion.
Accordingly, it is an object of the present invention to provide a target capable of generating a higher yield of nitrogen-13 than possible with the prior art.
It is another object to provide for the direct production of the desired chemical form of nitrogen-13 ammonium ion in a simple continuous flow collection which precludes complex chemical processing and radioactive losses.
It is another object of the invention, with the utilization of 10.5 MeV protons entering the target at a beam current of about 40 .mu.A, to produce about 100 mCi of nitrogen-13 ammonium ion in a time period of about 13 minutes.
A further object of the invention is to utilize the proton bombardment of a dilute natural water solution containing a solute that enhances conversion of nitrogen-13 atoms to nitrogen-13 ammonium ions with a lower production of nitrogen oxides.
It is also an object of the invention to utilize the proton bombardment of a dilute ethanol/natural water solution to produce nitrogen-13 ammonium ion.
These and other objects and advantages of the present invention will become apparent upon a consideration of the drawings and a complete description thereof that follow.